The present invention relates to a color night vision apparatus, and more particularly, to a color night vision apparatus employing multiple image intensifiers for viewing images in color during nighttime conditions or scenes under low light conditions.
Typically night vision goggles are monochrome output devices. In this manner, they take white light and input and amplify all colors and produce an monochrome display which is usually green due to the phosphors used. Several approaches to produce a color night vision system have been attempted. One technique uses two synchronized spinning color filter wheels in front and back of the image intensifier tube assembly. For example of this technique, reference is made to U.S. Pat. No. 5,162,647 issued on Nov. 10, 1992 to R. J. Field, Jr. and entitled, xe2x80x9cColor Image Intensifier Device Utilizing Color Input and Output Filters Being Offset By a Slight Phase Lagxe2x80x9d and is assigned to ITT Corporation, the assignee herein.
Essentially, the patent shows a color image intensifier device which uses an intensifier tube normally providing a monochrome output. There are input and output color members for passing respective different light frequencies. The input color member filters the incoming light to the tube of each light spectrum band in time succession and the monochrome output of the tube passes through an output color filter to produce a corresponding color component.
Another approach is to utilize spatial color filters in both the input and output of the image intensifier. These can be colored fiber optic cables or colored micro-lenses. This approach is a similar approach as analogous to operation in current cathode ray tube technology. The primary colors, for example, red (R), green (G) and blue (B) are sensed and displayed in different spatial positions adjacent to each other on a pixel level or scale. This produces a true color image. The above approaches all produce a direct view system.
Another approach is an indirect view system. In an indirect view system, input color filters such as a filter wheel, a sequential filter or micro-lenses are employed. The output of the image intensifier is then coupled directly to a CCD, or other imaging electronics. In the case of the spinning filter wheel, or the electronically tunable filter, the CCD is scanned at a rate appropriate to the filter. This produces three separate color images that are fused or combined together into a color image. In the case of the color fiber optical micro-lens, each pixel on the CCD corresponds to a different color. Adjacent pixels would then be fused into one color image. Depending on a filtering mechanism, these approaches can lead to true color images or false color images, if the near IR photons are filtered out.
As one can understand, it is desirable to have a color display for many reasons. For example, image pattern recognition is easier with a color image rather than a monochrome image. A color image is more desirable in surveillance applications, as it is desirable to use a color image intensifier device for consumer applications. Television news programs have shown scenes of and from image intensifiers to show news events and images during nighttime television operations. These images appear in a green color or in monochrome. These images are very different from the images one typically perceives viewing color television. A color image intensifier device prevents image loss during critical conditions by providing an output image in true color such as for nighttime medical care or for use in surgery.
For another example of a color image night vision device, reference is made to U.S. Pat. No. 5,233,183 to R. J. Field entitled, xe2x80x9cColor Image Intensifier Device and Method for Producing the Samexe2x80x9d and issued on Aug. 3, 1993 and is assigned to the assignee herein. In that patent, there is shown an image intensifier for producing a color output image, which has an RGB color filter matrix screen printed on a glass wafer which is laminated to the input faceplate of the tube and is sandwiched between the faceplate and the photocathode. The RGB matrix filters incident light into RGB components, which are amplified by the tube. The output image is colorized in a first embodiment by passing white light fluorescing from a phosphor layer through an RGB output filter matrix, which is aligned in an operating tube with the input matrix. In the second embodiment, a UV emanating phosphor layer excites an RGB matrix of secondary phosphors.
In any event, as is known, there are many different ways of producing color night vision displays in the prior art. A major disadvantage of prior art devices is that in order to produce the color image, the above mentioned approach separates the color signals in the white light in two distinct manners. Each of these approaches lead to some form of degradation of the system compared to optimum performance. The filters are time separation devices. Therefore, during the cycle when the red filter is in place, both the green and blue signals are attenuated. This leads to a reduction in the incoming signal by 66% if all time slices are equal for the colors. The 66% is derived from the fact that there are three primary colors involved. In addition, for electronically tunable filters, more light is lost in the transmission through the filters. Typically, these filters have a white light transmission value between 8 and 30%. This reduces the input signal to a range of 10 to 20%. Another disadvantage of electronically tunable filters is that these typically filter out the near IR portion of the spectrum which the image intensifier is fairly sensitive to. The reduction in the input signal results in poor level performance of the color night vision system. The micro-lens or color fiber optic approach separates the white light into colors spatially. The red, blue and green light are focused in immediately adjacent areas on the photocathode. This technique results in loss of signal, again in the range of 66% for the fiber optic and perhaps lower for the micro-lens. In any event, it does result in reduction of the overall resolution of the system.
In a color system, each monochrome pixel has to include three pixels, one for each primary color. This results in a loss in system resolution on the order of 30 to 40%. This is a loss in resolution if the pixels are of the same size as the monochrome pixel. In addition, each of the direct view approaches require the use of white phosphors. Typically, these white phosphors are less efficient than monochrome phosphors. The operation of the phosphor must be at a very high level to maintain the output brightness because the output signal is required to pass through the imaging filter assembly, and therefore reduces the output signal.
None of the above-mentioned approaches allow for the optimization of the image intensifier to specific portions of the spectrum which are supposed to be sensed.
A desirable approach would be to maintain the low light level sensitivity and the resolution according to the most modem efficient monochrome display. The problem of using a non-direct view system is the computer processing required to display the colorized image. In addition, current display devices which are small enough for head-mounted systems cannot and do not have the resolution comparable to the image intensifier system.
It is therefore an object of the present invention to provide a higher sensitivity and greater resolution color night vision system by retaining as much of the input signal light as possible while separating the colors spatially.
The resolution of the system is maintained because each color is imaged to a different spatial position to be amplified. The three color amplified image are recombined in a process which is the reverse of the input separation. The recombination forms a full color image. A true color image is provided with the addition of near IR blocking filter, otherwise a false color image would be provide which is red loaded. Due to the frequency or wavelength of infrared, this image would contain an excessive amount of red. By using three image intensifier tubes, each tube can be optimized for specific wavelengths of the spectrum allowing an improved lower light level performance.
A color night vision apparatus comprises at least three image intensifier tubes, each one associated with a different one primary color, and each tube having an input for receiving a low light image and an output for providing an intensified image. An input dichroic frequency splitter is positioned between the input of each tube and has an input port for receiving said low light image, and directs said input signal to said input of each tube, to cause each tube to provide an intensified output of said input signal and according to a primary color content in said input signal as associated with said respective tube, and having means responsive to said output signals of each tube for combining said signals indicative of each primary color to provide an output color image indicative of said low light image.